A biosensor (or biological sensor) is an analytical device including a biological element and a transducer that converts a biological response into an electrical signal. Certain biosensors involve a selective biochemical reaction between a specific binding material (e.g., an antibody, a receptor, a ligand, etc.) and a target species (e.g., molecule, protein, DNA, virus, bacteria, etc.), and the product of this highly specific reaction is converted into a measurable quantity by a transducer. Other sensors may utilize a non-specific binding material capable of binding multiple types or classes of molecules or other moieties that may be present in a sample. The term “functionalization material” may be used herein to generally relate to both specific and non-specific binding materials. Transduction methods used with biosensors may be based on various principles, such as electrochemical, optical, electrical, acoustic, etc. Among these, acoustic transduction offers a number of potential advantages, such as being real time, label-free, and low cost, as well as exhibiting high sensitivity.
An acoustic wave device employs an acoustic wave that propagates through or on the surface of a specific binding material, whereby any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Acoustic wave devices are commonly fabricated by micro-electro-mechanical systems (MEMS) fabrication techniques, owing to the need to provide microscale features suitable for facilitating high-frequency operation. Presence of functionalization material on or over an active region of an acoustic wave device permits an analyte to be bound to the functionalization material, thereby altering the mass being vibrated by the acoustic wave and altering the wave propagation characteristics (e.g., velocity, thereby altering resonance frequency). Changes in velocity can be monitored by measuring the frequency, amplitude-magnitude, and/or phase characteristics of the acoustic wave device and can be correlated to a physical quantity being measured.
In the case of a piezoelectric crystal resonator, an acoustic wave may embody a bulk acoustic wave (BAW) propagating through the interior (or “bulk”) of a piezoelectric material. BAW resonator devices typically involve transduction of an acoustic wave using electrodes arranged on opposing top and bottom surfaces of a piezoelectric material. In a BAW resonator device, three wave modes can propagate, namely, one longitudinal mode (embodying longitudinal waves, also called compressional/extensional waves), and two shear modes (embodying shear waves, also called transverse waves), with longitudinal and shear modes respectively identifying vibrations where particle motion is parallel to or perpendicular to the direction of wave propagation. The longitudinal mode is characterized by compression and elongation in the direction of the propagation, whereas the shear modes consist of motion perpendicular to the direction of propagation with no local change of volume. Longitudinal and shear modes propagate at different velocities. In practice, these modes are not necessarily pure modes as the particle vibration, or polarization, is neither purely parallel nor purely perpendicular to the propagation direction. The propagation characteristics of the respective modes depend on the material properties and propagation direction respective to the c-axis orientations. The ability to create shear displacements is beneficial for operation of acoustic wave devices with fluids because shear waves do not impart significant energy into fluids.
Although the preceding discussion has been focused primarily on biosensing and biochemical sensing applications in which mass is subject to being bound over an active region, BAW resonator devices may be used to detect various phenomena of interest, such as pressure in an environment containing an active region of a BAW resonator, density of a fluid medium arranged on or over an active region of a BAW resonator, and viscosity of a fluid medium arranged on or over an active region of a BAW resonator. Under an initial (e.g., no load) condition, an active region of a BAW resonator receiving an AC signal will vibrate at a natural resonance frequency. In exposure to a second condition (e.g., mass binding, pressure change, fluid viscosity change, fluid density change, etc.) that perturbs the active region (whether in a reversible or irreversible manner), the resonance frequency will shift, and a phenomenon of interest may be detected.
Temperature compensation in bulk acoustic wave (BAW) based liquid environment sensing applications is essential for creating a high-performance sensor with good sensitivity and a low limit of detection. This is because resonant sensors generally exhibit temperature sensitivities greater than the sensitivity of the phenomenon they are trying to detect. BAW-based sensor devices can operate in the 1-10 GHz frequency range with temperature coefficients of frequency (TCF) in the −20 to 0 ppm/° C. range for most aluminum nitride (AlN) based devices. For a device operating at 5 GHz with a −20 ppm/° C. TCF, a one degree change in temperature will bring about a −100 kHz shift in frequency. Often the frequency shift that is measured in liquid-based sensing systems is on the order of kilohertz, so temperature drift is clearly an issue that needs to be properly taken into account.
Temperature drift can be taken into account by three main methods: (1) maintaining a stable temperature environment in which the sensor will operate, (2) fabricating a device with 0 TCF at the frequency of interest, or (3) processing obtained sensor response data using methods that aim to subtract the temperature effects. The first and second methods can be hard to realize in practice, as a device with a TCF value even as low as 1 ppm/° C. TCF will experience a 5 kHz shift due to a 1 degree change in temperature when operated at 5 GHz. The third method traditionally requires either an independent measurement of the device temperature to be able to subtract the effect of temperature changes, or a separate reference device fabricated in proximity to the sensor. Use of a temperature sensor for temperature compensation, however, has multiple drawbacks, including: (i) introduction of an independent source of signal noise, thereby negatively affecting a net signal-to-noise ratio of a temperature compensated signal, (ii) introduction of a source of baseline drift; and (iii) increased cost and complexity of temperature measurement and data acquisition hardware.
Additionally, it may be challenging to select a phase crossing of a BAW resonator to be monitored as a function of time, particularly since a phase exhibiting maximum sensitivity may not necessarily provide an optimal signal in an environment in which temperature is changing.
Accordingly, there is a need for methods and systems providing improved temperature compensation and/or improved operational configuration of bulk acoustic wave resonators suitable for operation in the presence of liquid for biosensing or biochemical sensing applications in which temperature is subject to change, and that overcome limitations associated with conventional systems and methods.